1. Field of the Invention
This invention relates to an image recording/reproducing apparatus, such as a digital VCR (video cassette recorder) or the like.
2. Description of the Related Art
Recently, consumer-use VCR's, in which an image signal is subjected to high-efficiency encoding, and is recorded on or reproduced from a recording medium, such as a magnetic tape or the like, have been proposed. In such apparatuses, an image signal is divided into blocks, each comprising a predetermined number of pixels, and is then subjected to orthogonal transform, such as discrete cosine transform or the like. Coefficients after the transform are subjected to quantization/entropy encoding, and are recorded.
When variable-length encoding is performed in a VCR, it is desirable to make the amount of codes constant in each predetermined block, in consideration of particular reproduction, error propagation and the like. Accordingly, a method has been known in which the amount of codes is adjusted by adjusting quantization parameters of image data to be subjected to variable-length encoding.
FIG. 1 illustrates a format obtained by adjusting the amount of codes, having the configuration of product codes, in the above-described method using discrete cosine transform. In FIG. 1, the amount of codes in each of M (three in FIG. 1) sync blocks has a fixed length. A synchronizing signal (SYNC) and additional information (ID) are recorded at the head of each of sync blocks 1, 2 and 3.
As shown in FIG. 2, error-correcting codes, such as RS codes and the like, are added to data in each sync block, and the resultant data is recorded on a magnetic recording tape.
When reproducing recorded data to which error-correcting codes are added in the above-described manner, after demodulating a reproduced output from the magnetic tape, internal-code correction and external-code correction are performed, and the reproduced encoded data subjected to such error correction is decoded.
In the above-described VCR, the reproduced state from the magnetic head changes in accordance with a reproducing mode, such as normal reproduction or particular reproduction.
That is, in normal reproduction, helical tracks formed during the recording operation can be exactly traced in a reproducing operation. Hence, a stable reproduced output is obtained.
On the other hand, in particular reproduction, such as high-speed reproduction or low-speed reproduction, the relative speed between the magnetic head and the magnetic tape changes from that during the recording operation. Hence, the inclination of the scanning loci of the reproducing magnetic head does not coincide with the inclination of helical tracks, so that a stable reproducing state cannot be obtained.
In such a case in which a stable reproducing state cannot be obtained, the above-described error correction using error-correcting codes is not exactly performed, thereby causing erroneous correction.